The so-called "simulcast" methodology currently favored by the Federal Communications Commission as the basis for advanced television broadcasting in the United States, involves transmitting a conventional television signal, for example NTSC, over a first television channel, and transmitting a high definition television (HDTV) signal over an additional channel which would be assigned to each broadcaster. Since the assignment of an additional channel to each broadcaster will involve the use of those channels currently designated as "taboo" channels (i.e. those restricted for use in a given location), use of those channels will require that a way be found to prevent or minimize within acceptable limits, the interference caused by or to these additional broadcast signals with respect to the existing conventional signals.
The copending parent application describes a method and apparatus for implementing a digital television signal which will eliminate, or minimize to acceptable levels, interference to a conventional television signal present in a related television channel and which is least effected by the conventional television signal.
In order to accomplish its objectives, the parent application comprises a method and apparatus for digital source coding, channel coding, and modulating television information by grouping video data according to perceptual importance and placing the most important ("high priority") data in the part of the transmission channel which is least vulnerable to interference, and the least important ("low priority") data in the part of the channel which is most vulnerable to interference. Vulnerability can be defined in terms of interference from an NTSC signal provided on a co-channel (i.e. "taboo" channel) or the susceptibility of a signal component to drop out due to reduced signal strength.
As with the parent application, the instant invention comprises source coding which can provide, for example, a data stream having a data rate in the vicinity of about 20 Megabits per second (Mb/s). After appropriate multiplexing of digital audio, other digital data (i.e. control signals), and forward error correction and channel coding, the data rate presented to a modulator is nominally about 30 Mb/s. Such a data rate requires a modulation scheme which can deliver nominally 5 bits/sec/Hz in order to transmit this information in a 6 MHz channel. A preferred type of modulation is quadrature amplitude modulation (QAM), for example 64-QAM (64 quantizing levels).
The multi-carrier embodiment of the invention described in the parent application includes means to suitably shape the spectrum of the modulated signal in order to provide different signal-to-noise ratios for different carriers. This provides the flexibility of conveying different types of information (e.g. different codewords or different bits) via different carriers to the decoder. A few examples of relatively high priority information that could be transmitted with low probability of error are motion vectors, the low-frequency coded video (both luminance and chrominance), and intra-frame video coded information (the refresh information). On the other hand, the information with lower priority can consist of high-frequency components in the refresh frame, and the motion-compensated frame-difference information for both luminance and chrominance.
Digital video compression system specifications known as MPEG (Motion Pictures Experts Group) are being developed by a joint ISO/IEC committee. Several digital television systems are being proposed for use in broadcasting, cable television and multimedia applications (for example Compact Disc Interactive) which incorporate and will be compatible with the eventual MPEG standard(s). A concise review of the proposed MPEG specifications is provided by Woodward in his article "An Overview of the JPEG and MPEG Video Compression Specifications" which appeared in 1991 NCTA Technical Papers, pp 135-141, and which is incorporated by reference herein.
As described in the Woodward article, three types of video frame coding are used in accordance with MPEG: intra-coded frames (I), predictive coded frames (P) and bidirectionally predictive coded frames (B). I frames are coded independently from the other frames. They serve as anchors to the P and B frames. The P frames are temporally predicted from previous anchor frames (either I or P frames), and the B frames are interpolated (and/or predicted) from two adjacent anchor frames (either I or P frames). A review of intraframe and interframe video coding is provided by Feher in his book "Advanced Digital Communications", Prentice-Hall, Inc., Englewood Cliffs, N.J. (1987) which is incorporated by reference herein.
In accord with the MPEG protocol, the frames are divided into blocks. A "block" is an orthogonal 8 pel by 8 line section of luminance or chrominance components of an image. A "macroblock" is a 16 pel by 16 line section of the image containing four luminance blocks, two each in the horizontal and vertical directions, and two corresponding chrominance blocks, one for each component. A "slice" is a series of macroblocks in the raster scan order. Macroblocks form I,P or B frames will be described herein as I,P or B macroblocks respectively. A more detailed discussion of these and other details of the MPEG protocol is provided in the publication of the International Organisation For Standardization entitled "MPEG Video Committee Draft", published December, 1990 and a further draft dated May 31, 1991, both incorporated by reference herein.
The slices are coded into bits, grouped into packets and can be transmitted, as discussed in the parent application, as either high priority data or low priority data, the low priority data being more susceptible to bit errors which can occur for example, as a result of noise, timing jitter or electrical interference. The designation and handling of information derived from the macroblocks, as either high priority or low priority, depends upon the degree of importance of the information to reconstructing a good quality image. I frames are accorded the highest priority, and are therefore protected from the effects of errors to a greater degree, since they are the "refresh" frames and provide information from which the B and P frames are predicted. B frames are accorded the lowest priority, and are therefore protected from errors to a lesser degree. Any degradation due to error in a B frame persists only for one frame.
Error correction techniques such as those described in the parent application have, only a limited ability to correct most of the bit errors which can occur either singly or grouped as bursts. Due to the data compression which takes place during the channel coding, even a single uncorrectable bit error can result in the loss of a considerable amount of video data since the entire packet will be effected (and discarded) effecting a number of macroblocks. If a detected error cannot be corrected, it is necessary to replace the missing information in some fashion, i.e. to "conceal" the error.